In recent years, the trend toward higher processing speeds, the shortage of organic solvents, air pollution regulations and the energy crisis have all served to increase the already substantial interest in high-solids coatings and inks that can be cured by brief exposures to radiation. These systems normally contain reactive monomers rather than inert solvents, and cure by the radiation-induced polymerization of the ink or coating composition.
As this concept has grown from the research stage into commercialization, the technology has been applied to a wide variety of situations, i.e., a host of different substrates have been coated by means varying from roll coater to lithographic press, and cured by means varying from electron beam to ultraviolet light to heat. The radiation-induced free radical polymerization or cure often results in some volume shrinkage, with the actual amount of shrinkage varying from system to system. This shrinkage in many instances results in poor adhesion, particularly to rigid substrates.
One approach to solving the adhesion problem has been the inclusion of vinyl resins. The excellent performance of vinyls on rigid substrates has been recognized for many years--however, difficulties were encountered in incorporating them in acrylate-radiation curable systems. High viscosities were encountered when only small amounts of conventional vinyl resins were dissolved in the reactive monomers.
Lower molecular weight vinyls were prepared, and were found to be considerably more useful. At a given viscosity, it was observed that considerably more vinyl could be incorporated, and adhesion to many substrates was improved. This information was the basis for U.S. Pat. No. 3,943,103. The only problem associated with the use of these materials is the necessity of formulating very, very carefully when solvent resistance is desired.